Polythiophene has many potential applications including organic solar cells, smart window systems, organic light-emitting diodes (OLEDs), transparent electrodes, antistatic agents, etc.
Although polythiophene-based conductive polymers are generally synthesized by a metal-catalyzed crosslinking reaction, the Grignard metathesis (GRIM) method provides greatly improved electronic and optical properties as compared to the existing random regioisomeric analogues owing to the high regioregularity of the polythiophene side chains arranged head-to-tail (HT).
It is known that the synthesis of polythiophene-based conductive polymers by the GRIM method allows control of not only regioregularity but also molecular weight and molecular weight distribution to some extent, which can be achieved through quasi-living polymerization of thiophene. Living polymerization is advantageous in that molecular weight can be predicted from the amount of monomers and initiator used because of fewer side reactions such as termination during the polymerization, and that, since the obtained polymer has a narrow molecular weight distribution and has living ends (livingness), the molecular weight can be increased further by adding more monomers. Accordingly, the method can be used to synthesize block copolymers. In the GRIM method, a Ni(II) catalyst is used as an initiator to control the molecular weight. Depending on the type of initiator used, living polymerization is classified into living radical polymerization, living anionic polymerization and living cationic polymerization.
In general, a star polymer refer to polymer having a three-dimensional structure wherein several linear polymer chains are connected to a central core by chemical bonding. When compared with linear polymer having the same molecular weight, a star polymer exhibits very low viscosity and very superior solubility because of compact structure and small hydrodynamic volume.
In general, conductive polymers like polythiophene are difficult to be synthesized with a large molecular weight because of poor solubility resulting from a rigid structure wherein aromatic compounds or olefin compounds having resonance structure are connected to a main chain. Although solubility has been increased by introducing soluble alkane groups to the side chains to overcome this problem, the insulating alkane groups disadvantageously lead to decrease in conductivity resulting from the main chain having the resonance structure.
Whereas synthesis and physical properties of linear polythiophenes having two-dimensional structure have been researched, there have been few researches on the synthesis and physical properties of polythiophenes having three-dimensional structure such as star polymers.
In general, polythiophene exhibits very low electron mobility (10−8 S/cm) in the absence of a dopant. When doped through oxidation or reduction, the electron mobility of polythiophene is increased to about 102 S/cm. At present, PEDOT:PSS using polystyrene sulfonate is the representative dopant used for polythiophenes and it is widely used, for example, in a buffer layer (hole transfer layer) of an organic solar cell. Although PEDOT:PSS is advantageous to laminate organic materials because it is dispersed in water, it has processability problem because it is not soluble in organic solvents. In addition, the presence of a water-absorbing layer negatively affects device performance in the long term.